1. Field of the Invention
The present invention relates to apparatus for preparing a tubular parison that is continuously, downwardly extruded from an extrusion head and for transferring sections of the parison to a blow molding machine to be formed into articles.
Processes are known for blow molding articles from, for example, molten thermoplastic materials that are downwardly extruded as a tubular parison from an extrusion head. Typically, the parison is divided into discreet sections that are sealed at opposite ends to confine therein a volume of air at pressure slightly above atmospheric pressure. This slightly pressurized air prevents the side walls of the parison from collapsing into contact with each other. Each parison section is then clamped between mating die members in a blow molding machine and air at higher pressure is injected into its interior to expand it against the faces of mold cavities, defining the external shape of the article being made, formed in the mating die members. The molded article is then permitted to cool for a short period of time and is thereafter removed from the mold.
Problems in achieving optimal extrusion and molding efficiency have been encountered in the past because the extrusion process is continuous, but the molding process is not.
Problems in blow-molding articles from tubular parisons have also existed in the past because of difficulties in controlling parison wall thickness. Various factors operative during the extrusion process influence the wall thickness of the parison and often combine to render the wall thickness non-uniform. For example, it has been found that thermoplastic material expands or swells after passing through an extrusion head under pressure. Further, after the thermoplastic material has been extruded from the head, it moves downwardly pushed by other material from the head and pulled by gravity. The longer and heavier the parison becomes, the more material immediately adjacent the extrusion head is elongated. Moreover, the material first extruded from the head is subject to longitudinal contraction through cooling and elastic recovery at an earlier time than subsequently extruded material.
It has been found that these factors can cause a parison extruded from a die head with an intended wall thickness of 0.1 inch to, in fact, have a wall thickness of 0.110 inch at its lower end and of 0.050 inch at its upper end adjacent the extrusion head. Such a parison is unsuitable for molding articles where uniform wall thickness is essential.
2. DESCRIPTION OF THE PRIOR ART
Various proposals have been made in the past to meet problems which arise because of the continuous nature of the parison extrusion process and the discontinuous nature of the molding process. Apparatus in accordance with one such proposal includes a molding machine having a plurality of mating die members that are serially moved into position to receive an extruded parison section. While one parison section is molded into an article between mating die members, a second parison section continues to be extruded from the extrusion apparatus. The second section is then molded while the first molded article is stripped from, for example, the first mating die members which are thereafter again ready to receive another extruded parison section.
Another proposal involves mounting the molding machine to move into the proximity of the extrusion apparatus to receive an extruded parison section. Once the mating die members are closed about the parison section, the molding apparatus is moved to a position remote from the extrusion apparatus where the molding process is completed and while a second parison section continues to be extruded.
Apparatus of the types generally described above are not entirely satisfactory because movement of the molding machine requires relatively complex equipment. Moreover, the machine usually has large mass and, therefore, large amounts of energy are needed to move it between molding and parison receiving positions. Movement of the molding apparatus is also time-consuming.
Proposals have also been made in the past for transferring extruded parison sections to molding machines mounted at locations remote from an extrusion apparatus. In one such machine, the blow-molding machine is mounted directly beneath the extrusion apparatus by a distance sufficient to permit a full length parison section to be extruded above the molding machine. The section is then severed from the remaining continuously extruded parison and is transported downwardly into the molding machine where it is formed by mating die members into an article. Extrusion of the parison continues above the molding machine during the molding process. U.S. Pat. Nos. 3,504,396 (Button et al.) and 3,663,672 (Button et al.) disclose an apparatus and method that operate using this principle.
In order to overcome the problems, noted above, of producing a parison having uniform wall thickness, stretching mechanisms have also been proposed. Some such mechanisms stretch the parison in regions where uncontrolled extrusion results in increased wall thickness and support the parison against gravity in regions where decreased wall thickness ordinarily results. However, for reasons which will be explained below, this approach is not entirely satisfactory.